BRITISH actor Charlie Hunnam is set to play billionaire Christian Grey in the big-screen adaptation of saucy novel Fifty Shades Of Grey.

The 33-year-old, who most recently starred in Guillermo del Toro's sci-fi film Pacific Rim, will take on the role of the wealthy business magnate, who seduces innocent college graduate Anastasia Steele in the upcoming film.

The book's author EL James, whose real name is Erika Leonard, told fans on Twitter: "The gorgeous and talented Charlie Hunnam will be Christian Grey in the film adaptation of Fifty Shades of Grey."

US actress Dakota Johnson, the 23-year-old daughter of actors Don Johnson and Melanie Griffith, has signed up to play the lead role of Anastasia.

"I am delighted to let you know that the lovely Dakota Johnson has agreed to be our Anastasia in the film adaptation of Fifty Shades of Grey," James wrote.

Dubbed "mummy porn", the novel featured sado-masochistic sex and became the fastest-selling paperback of all time.

Hunnam is best known for his TV role as Jax Teller in US series Sons Of Anarchy and gay teenager Nathan Maloney in the Channel 4 drama Queer As Folk, while Dakota's roles to date include appearing alongside Justin Timberlake in The Social Network and starring in the short-lived US TV series Ben And Kate.

Dakota has signed up to play the lead role of Anastasia Steele (Image: Getty Images)

Former Harry Potter actress Emma Watson, model Cara Delevingne and Game of Thrones star Emilia Clarke are among the actresses who have previously been linked to the role.

Artist turned filmmaker Sam Taylor-Johnson is directing the big-screen version of the smash hit erotic novel, which is scheduled for release in cinemas in August 2014.

Her husband, Kick-Ass star Aaron, has ruled himself out of the role of Christian Grey, saying that he is too young. Other actors who had been rumoured for the part included Ryan Gosling and Ian Somerhalder.